1. Field of the Invention
The invention relates to the field of solid state imaging devices utilizing diffused floating gates.
2. Prior Art
In the prior art, solid state imaging devices are well known and detailed. Vidicon tubes as imaging devices are perhaps best known and commercially acceptable because of the advantage of operation in the charge storage mode. In this device a charge is stored on each imaging element for the full time frame between the interrogating intervals and the output signal represents the stored charge remaining after the frame time interval. The application of this principle to solid state imaging devices has provided advantages of a similar nature but has not solved the low signal-to-noise level inherent in these devices.
An early summary of solid state imaging devices is found in IEEE Spectrum, March, 1969, pp. 52-65 and IEEE Transactions on Electron Devices, April, 1968, Vol. ED-15, No. 4, pp. 256-261. But, since all of the multi-element imaging systems disclosed include a multitude of internal capacitances coupled by way of a common substrate or by connections to the image element being interrogated, the output signal is accompanied by a high level of noise representing the capacitance coupled signals from the non-interrogated elements. The noise level stems from the capacitance coupled non-interrogated elements and occurs during switching from element to element, which coinciding with the signal, maintains the noise level high.
To overcome this problem, the prior art has tried a variety of techniques utilizing a number of photo-sensitive elements. Representative is the patent to Shannon, U.S. Pat. No. 3,721,839, which utilizes a junction field effect transistor (J-FET) with circular drain surrounded by a circular junction gate with the epitaxial layer serving as a common source for a line of elements. A pulse is applied to the gates to deplete the channel around the drains, and a non-destructive read-out is obtained by measuring the impedance between the drains and a common source. In Shannon, response is controlled by varying the epitaxial layer-substrate reverse bias as a function of depth of the depletion region. The gates in this patent are driven by other devices and are directly coupled by metal contacts. The number of metal contacts required is a disadvantage of prior art imaging devices such as shown in Shannon because making metal semiconductor contacts is done at the expense of image resolution. That is, space is required to make each contact resulting in a decreased packing density of sensors.